Many filtration applications require the removal of particles in the micron and submicron ranges from a fluid medium. Microporous filtration media employed for such purposes are typically relatively delicate structures which are easily damaged. For example, U.S. Pat. No. 4,431,545 discloses a hydrophilic, microporous filter system having ultrafiltration capability, i.e., the ability to remove particles as fine as about 0.001 micrometer up to about 10 micrometers. A preferred filter medium for use in the system of U.S. Pat. No. 4,431,545 is a thin, typically a few mils thick, microporous polyamide membrane. This membrane is difficult to work with because of its limited strength and its lack of internal rigidity. In many other similar uses, the microporous membranes used are also thin, relatively delicate structures with little internal rigidity and very low flexural moduli, i.e., they do not retain their shape when unsupported, typically hanging limply in loose folds, much as a piece of thin cloth drapes when unsupported.
The combination of their relatively delicate nature (limited strength) and lack of internal rigidity (low flexural modulus or modulus of flexure) creates difficulties in working with such membranes, e.g., in corrugating them to increase the surface area available in a filter element or the like.
One approach used to overcome these difficulties is to cast the membrane, e.g., by the method described in U.S. Pat. No. 4,340,479, onto a substrate, such as a fibrous web or mat, which is incorporated into the membrane and becomes a permanent part of the structure, thereby enhancing both the strength and the flexural modulus of the membrane. Offsetting the desirable increase in strength and flexural modulus is an undesirable increase in the pressure drop of the membrane.
For example, polyamide membranes having an absolute pore rating of 0.2 micrometer are commercially available from Pall Corporation under the trademark ULTIPOR. For an unsupported (i.e., no internal fibrous mat support) double layer membrane of this type, the pressure drop at an air flow rate of twenty-eight feet per minute is nine inches of mercury. For an equivalent supported, double layer membrane with the same pore rating, the pressure drop is twenty-five inches of mercury, nearly a three-fold increase. This pressure drop difference is highly significant in the operation of an on-line filtration system. For example, in the pharmaceutical industry, efficient, fast filtration at reasonable pressure levels is often important in the processing of sensitive compositions such as parenterals. With the particular polyamide membranes described above, the time to filter a typical pharmaceutical industry fluid at a specified pressure would be 2.8 times longer for the supported membrane with clean membranes and relatively low contaminant levels in the fluid being filtered. For this reason, in many applications the unsupported membrane is highly preferred. Offsetting the advantage of lower pressure drops with concomittant higher filtration rates at comparable pressures is the difficulty in working with the delicate, low strength, low flexural modulus of the unsupported membrane. Care must be exercised in handling the delicate material since it is relatively easily damaged and any form of crack or tear, even a minor one, will render such material essentially useless as a filtration medium.
The difficulty of working with such delicate materials is further evidenced by the commercial unavailability in flat disc form of thin, fine pored, polytetrafluoroethylene (PTFE) membrane material--a highly desirable filtration medium for many applications. In addition to having a low flexural modulus and limited strength, PTFE and similar membrane materials have a tendency to become electrostatically charged. Such a material sticks to itself in a manner making it very difficult to maintain in flat or planar disc form. Additionally, because of the low flexural modulus and the difficulty in sealing PTFE membrane to support materials, e.g., a polypropylene housing, it is difficult to manufacture filter structures incorporating this type of membrane in such housings. For example, in biomedical applications it is often desired to insert a precut flat or planar filter piece into a preformed support structure and tightly seal the periphery of the filter piece to the support structure. Unsupported PTFE can not readily be used in such a manner because of its low flexural modulus, which makes accurate placement troublesome, and the difficulty in sealing it to the support structure.
The subject invention is directed to supported microporous membranes which substantially overcome the difficulties described above in working with such relatively low strength, low flexural modulus membranes. Indeed, as described in detail below, the supported microporous membranes in accordance with this invention also provide additional advantages in certain filtration applications.